During well completion of a fully drilled oil and/or gas well, a number of casings of different lengths and diameters will be cemented to the ground formation. Between the casings, which are disposed coaxially with each other, a so-called annulus will be formed. To prevent a leakage in the oil and/or gas well, a plurality of packer elements will suitably be arranged between the casings. The casings will be suitably suspended from a wellhead structure, where the wellhead structure is arranged at the top of the oil and/or gas well. During operation of the oil and/or gas well, the wellhead structure will conduct the well stream therethrough for further processing of the well stream. The wellhead structure will also be a safety mechanism against the well stream flowing uncontrolled to the surface.
A wellhead structure of this kind is subjected to large loads and stresses from the surrounding environment. Although these structures and installations are designed to be maintenance-free for a number of years, they must be inspected constantly for safety and financial reasons.
It is both desirable and necessary to carry out an inspection of such offshore installations, for example, various equipment, pipelines, wellheads etc., not only during production, but also during drilling, installation and maintenance and repair work, this inspection taking placed in the form of automated operations. This means that quite different demands are made on the equipment and monitoring, inspection and communication systems that are used offshore than what is normal for installations onshore.
In addition to the above, it will be extremely important to know how an oil and/or gas well is behaving, or what is happening in the oil and/or gas well, and this will be the case throughout the entire lifetime of the well, i.e., from when the actual drilling of the well starts until the well is finally shut down. This is done by monitoring a number of different parameters in the well, which parameters may for example be contamination, leaks, well pressure, the production itself, sand/erosion in the well, wellhead temperature, the state or condition of various equipment (for example, the position of a valve), corrosion etc.
In connection with, for example, production of oil and/or gas wells, it will be extremely important from a safety, reliability and cost aspect to prevent a so-called pressure leak from the well through the different annuli in the casings, and out to the surroundings. If an undesirable pressure leak of this kind nevertheless occurs, various safety systems are intended to be able to close the well even under pressure, so that well fluid which has flowed into the different annuli of the well can circulate out in a controlled manner.
By constantly or repeatingly carrying out measurements of, for example, the pressure in the well, where this can be done at a number of different points in the well, it will be possible to have at an earlier point in time an indication that a pressure increase is about to occur in the well, that a pressure leak in the well will or has already occurred, whereby various actions can be taken to ensure that the consequences of such a pressure build-up will be minimal or to prevent them altogether.
Various solutions have therefore been developed to monitor and/or control pressure in an oil or gas well, Reference can be made, for example, to U.S. Pat. No. 5,172,112, in which there is known that a pressure-measuring device measures pressure in a subsea pipe. The device includes a stationary unit mounted to the exterior of the subsea pipe and a movable unit that is lowered into position next to the stationary unit whenever the pressure is to be monitored or measured. The stationary unit, which is a strain gauge, will monitor the pressure in the pipe by measuring the “strain” in the pipe. The measurements will subsequently be transmitted from the stationary unit in the form of suitable signals, whereby the movable unit will then convert these signals to give a picture of the pressure that is within the subsea pipe.
A solution is known from GB 2 286 682 where an inductive pressure transducer is used to measure the pressure within a pipe. This is accomplished by passing an alternating current within an inductor coil to generate a magnetic field. The magnetic field passes through a gap formed between the pipe and the inductor coil, and then into the pipe. The fluid flowing in the pipe will, owing to its pressure, induce stress in the pipe, which stress will cause variations in the electromagnetic properties of the material from which the pipe is made, which variations can be sensed by the magnetic field that is formed. The sensed variations can then be converted to give a pressure measurement.
Another system for detecting a leakage in an oil and/or gas well is described in U.S. Pat. No. 4,116,044, where the system comprises a plurality of pressure-sensitive transducers that are arranged in a through hole in a wellhead. The pressure-sensitive transducers will be so arranged that they can detect a leakage in a plurality of annuli in the well. The transducers are connected through wires to a junction box which will be capable of carrying signals to a processing location. During replacement of the transducers, the well will have to be shut down as the replacement operation will involve the well being “opened”.
It is an object of the present invention to provide an improved apparatus for monitoring physical parameters in an oil and/or gas well, for instance with regard to safety, including fire safety, reliability and/or costs.